The present invention relates to electronic devices. In particular, the present invention relates encapsulation materials for producing electrical interconnections.
Solder joints are commonly used for electrical interconnection for electronic devices. In flip chip technology, integrated circuit chips (IC chips) are electrically connected via solder bumps to the input/output (I/O) pads of the substrate. this is done through a solder bumping process in which interconnections between the IC chip and the corresponding substrate is made by melting or reflowing solder bumps placed on the prescribed input/output (I/O) pads of the IC chip. In order to achieve reliable solder interconnections between an IC chip and substrate, or between two substrates, a fluxing agent is typically applied to the I/O pads to rid them of any oxides, because the presence of oxides affect the wetting of the metallic I/O pads, resulting in weak joints upon attachment. Following proper soldering, the fluxing agent is removed by washing, after which an filling material is applied to fill the gap between the IC device and the substrate in order to protect the package from damage. This material is generally known as underfill encapsulant material.
The encapsulant material is generally made from thermoset plastics such as epoxy. There are 3 general types of epoxy resin: bisphenol type, cycloaliphatic type and novolac type. The chemical characteristics and curing reactions are generally disclosed in Handbook of Epoxy Resin, by Lee, H. and Neville K., McGraw Hill Book, New York, 1987.
It is known to use an underfill encapsulant material that contains a fluxing agent in order to reduce the number of steps involved in packaging. Using this xe2x80x9cno-flowxe2x80x9d underfill, the separate fluxing step and the subsequent cleaning steps are eliminated. Instead, the xe2x80x9cno-flowxe2x80x9d underfill combines the fluxing step and the encapsulation steps together, as disclosed in U.S. Pat. No. 5,128,746.
WO 99/03597 further discloses the use of an acrylic or methacrylic acid as the fluxing agent in combination with a neutralisation agent. This fluxing agent is optionally combined with a resin adhesive system to form a thermally curable adhesive composition. Organic acids are commonly used as fluxing agents, but the kinetics of their redox activity in combination with the curing kinetics of the adhesive is not well characterised. The fluxing agent must maintain good fluxing activities before and during the course of soldering, and the rate of curing must be properly controlled such that hardening occurs only after the proper fluxing and soldering has been completed. However, the uncured encapsulant should have a relatively long storage life for convenience of use. Furthermore, it would be ideal for the adhesive to be reworkable. Lastly, the fluxing agent should not have residual activities that may affect the subsequent solder joint.